1. Field of the Invention
This invention relates to a compact of hydrogen adsorption alloy principally composed of metal hydride and, more particularly, to a compact the hydrogen adsorption capacity of which is hard to to lower or decline in spite of repeated uses.
2. Prior Art
Heretofore, several arts have been developed with respect to the hydrogen adsorption alloy, wherein a hydrogen is adsorbed in a certain metal alloy to be stored therein and transferred therefrom in the form of a metal hydride. These arts have been further applied to such practical uses as the purification of hydrogen, the design of heat pumps, the design of air-conditioning systems, etc.
In such cases, since an exothermic reaction or an endothermic reaction necessarily takes place when the metal hydride adsorbs or discharges the hydrogen, the delivery of hydrogen does not take place without rapid delivery of heat between the metal hydride and the outside.
However, a thermal conductivity of the hydrogen adsorption alloy itself is actually low and is further lowered by decay and micronization of the alloy along with repeated and continuous adsorptions and discharges of the hydrogen, bringing about the lowering or decline of the hydrogen adsorption capacity.
In order to overcome this problem, to date several attempts have been proposed. For example, according to the "Method of Improving a Heat Conductivity of Metal Hydride" disclosed in Japanese laid open Patent Publication (unexamined) No. 56-120502, a metal hydride is divided finely into particles to fill in cells of sufficiently heat-conductive porous material which are communicated with the outside thereof, and the heat is supplied or exhausted by way of this material. It is reported that by the foregoing design, the heat conductivity of the metal hydride is improved.
Further, according to the "Compound Compact of Hydrogen Adsorption Alloy and Manufacturing Method thereof" disclosed in Japanese laid open Patent Publication (unexamined) No. 59-162102, fine particles of hydrogen adsorption alloy are mixed with a powdered or fine fibrous heat conductive metallic material, and a process of sintering is applied to a mixed material within the temperature range higher than the melting point thereof. A compact is formed thereby in such a manner that the hydrogen adsorption alloy is included in a two or three dimensional network (or meshed) structure of the heat conductive metallic material. It is reported that a measured value of an effective thermal conductivity (W/m.K) of this compact amounts to 30 to 100 times as many as the powdered hydrogen adsorption alloy layer.
It is further reported according to Thyssen in German Pat. No. DE 3144913, that partially coating surfaces of powder particles with a metal softner before these powder particles are to be molded does not appreciably promote bonding of the powder particles as well as compactibility by compression molding. According to Thyssen, the coating should be applied partially to the powder particles so as not to effect a reactivity with hydrogen gas.
A serious problem which is common in all of the above-mentioned three known arts exists in that the powdered hydrogen adsorption alloy material itself is not improved at all. In other words, at the earlier stage of use, both of the known arts assure excellent heat conductivity of the compact as a whole by the action of the heat conductive metallic material enabling rapid adsorption and discharge of the hydrogen, but along with repeated and continuous uses, the surface of the hydride becomes contaminated (or corroded) by impurities in the atmosphere or oxided by the oxygen in the air to the extent of forming an oxide film, thereby considerably declining the adsorption function. Moreover, as a result of volume expansions and contractions repeately taking place for every hydrogeneration and dehydrogeneration, the compact decays and is micronized, finally getting over the constraining force of the metallic matrix and the micronized particles dropping out thereof.
Thus, a first problem to be solved is that the adsorption capacity declines is a short period caused by a decline of heat conductivity and filling rate along with continuous use.
Furthermore, in the case of the foregoing first prior art, since the fine particles of metal hydride of 50 meshes or so are simply infiltrated into the porous cell by vibration and no other particular process is applied thereto, the fine alloy particles are easily micronized by repeated use and drop out of the cell, eventually bringing about the lowering of the adsorption capacity.
In the case of the foregoing second prior art, since it is required that the sintering process is applied to the heat conductive metallic material within a higher temperature range than the melting point thereof, a mixture of the powdered hydrogen adsorption alloy (MmNi.sub.4.5 Al.sub.0.5) with the powdered copper formed by compression molding is to be treated with heat at 1100.degree. C. for 4 hours according to one mode of this prior art. In applying such a heat treatment at such a high temperature, it is essentially required to use a heat-treat furnace which is controllable with high accuracy, and the level of control and operation thereof must be also kept high.
According to Thyssen, reactivity is not affected by partial coating. However, neither thermal conductivity nor compactability by compression molding are promoted. As a result, with repetition of reactions, there is the possibility that the molded compact is micronized or deteriorated in a rather short period of time.
Moreover, in both of the foregoing prior arts, when it is necessary for a compact to be drilled as an integral part of an apparatus, since the constraining force of the metallic material used as a base material (or a cell) is small, a sufficient workability is not secured and even when continuing the drilling work, the finish thereof is rough and poor.